This invention relates to the art of earth boring and more particularly to an earth boring machine that produces a bore in earth formations by loosening portions of the formations at the face of the bore.
This type of earth boring machine is characterized by a rotary head connected to the body of the machine that includes cutters for contacting the face of the bore and loosening material at the face of the bore. The loosened material falls to the bottom of the bore where it accumulates and must be picked up for removal. Conventionally, buckets attached to the rotary head scoop up the loosened material and transfer it to a conveyer and/or other equipment for removing it from the bore. The buckets should be in a position proximate the bottom wall of the bore when they are scooping up the loosened material. At other locations throughout the rotation of the rotary head, the buckets need not be proximate the wall of the bore.
Under some circumstances it is undesirable for the buckets to remain proximate the wall of the bore throughout the rotation of the rotary head. For example, when boring through caving formations, it is desirable to provide a shield above the machine to prevent the caving materials from falling onto the machine. The shield should extend to a point near the rotary head. If the buckets are proximate the wall of the bore in the vicinity of the shield, the shield must be spaced from the rotary head. This allows portions of the caving material to fall onto the machine through the space between the rotary head and the shield.
It will be appreciated that it may be desirable to have the buckets positioned at a shorter radial distance from the central axis of the rotary head during a portion of the cutterhead's rotational cycle to prevent the buckets from interfering with elements other than the previously mentioned shield. Space is extremely valuable on an earth boring machine and available space is needed for elements of the earth boring machine or companion equipment. For example, lining systems are often used in conjunction with tunneling machines, and it may be desirable to place some of the elements of the lining system close to the rotary head.